Track systems have been commonly employed for sectional doors for many years to control the movement of the individual sections of the doors. Common examples of such sectional doors are the type employed as garage doors in homes, commercial and utility buildings, and similar applications. These track systems are commonly of a generally L-shaped configuration, which includes a vertical section positioned proximate a door frame and a horizontal section that extends substantially perpendicular to the vertical section rearwardly into the interior of the building in an overhead position. A curved transition section is normally positioned proximate the header at the top of a door frame and interconnects with the vertical section and the horizontal section to provide a continuous track system. The panels constituting conventional sectional doors are joined by hinges on the inner or internal side of the door, which form pivot points at the juncture between each of the panels to permit the panels to pivot inwardly toward each other as the door progressively moves through the curved transition section between the vertical closed position and the horizontal open or storage position. The hinge brackets normally have projecting flanges which mount rollers that are captured in the track sections, which are normally of a "J" or "C" cross-sectional configuration.
In a great many environments, there are severe limitations in the overhead clearance available above a door frame, which may take the form of the garage ceiling or support beams, piping, or other elements which extend downwardly from the ceiling. In order to minimize the overhead clearance required for the horizontal track section and the operation of the door panels in relation thereto, it is common to place the curved transition section of the track as low as possible and preferably in the area of rather than above the header of a door frame. As a result, the mounting bracket for the roller positioned proximate to the upper edge of the top panel of the door to control its movement must be offset a substantial distance from the door to repose in the curved transition section in order to provide for full closure of the upper panel of the door. This, however, results in the roller at the top of the top panel being offset from the door a substantially greater distance than the other rollers.
While this arrangement has been widely employed in the industry for many years, it produces a problem when the sectional door is moved to the open position as the rollers move into the horizontal track section. In particular, with the door supported on the rollers in the horizontal track section, the greater offset of the top roller than that of the roller at the juncture between the top panel and the upper middle panel and the roller at the juncture between the upper middle panel and the lower middle panel, urges the top and upper middle panels to pivot at their juncture in a backward or reverse direction to the hinge connection. This condition, which is commonly aptly termed "backbreak" in the industry, is a source of wear that can lead to premature failure of components. In this respect, there is a tendency for the edges of the top panel and the upper middle panel to be crushed by the backward compressive force which is contra to and cannot be accommodated by the hinges. Further, the edges of the top panel and the upper middle panel in the area underlying the hinge tends to separate, which can produce damage to the hinge, the hinge pin, the fasteners attaching the hinges to the door panels, and/or the integrity of the door panels themselves. Additionally, the rollers between the top panel and the adjacent or upper middle panel can be forced upwardly, despite the weight of the door, so severely as to distort, even permanently, the horizontal track section and the rollers. In extreme cases, the rollers may distort the track section to an extent that a roller escapes from the track, causing a system breakdown. At the least, this condition causes erratic resistance to door movement and attendant increased noise levels.
The industry has tried numerous approaches to endeavor to eliminate or at least reduce the severity of this backbreak condition. In some instances, the extent of offset of the top roller of the top panel is reduced; however, in this circumstance, there is normally a difficulty in seating the top panel against the door frame, such that the panel may rattle or vibrate and is not sealed in weather-tight relation to the door frame or weather stripping which may be installed thereabout.
Another approach which has been taken in some instances is to lower the position of the top roller on the top panel, such that a lesser offset distance is required. While this may somewhat reduce the backbreak condition, it creates a high arc condition in that the top of the top panel moves a greatly increased distance above the horizontal track section as it passes through the transition section. Thus, substantial additional headroom is required to clear the travel of the top of the top panel, which, as previously indicated, is a severe limitation that must be avoided for all-purpose door installations which seek to meet stringent overhead clearance limitations.
Approaches which have been directed to a departure from the basic door and track configuration have suggested the use of a separate track for the top roller; however, this entails a much more complex and expensive track arrangement and loss of headroom. Other approaches have included the construction of track sections of heavier gauge material to prevent deflection and possible distortion due to the movements of the rollers; however, this solution merely concentrates the resultant forces on the rollers, hinge brackets, and the door panels themselves.
Due to the lack of a viable solution, the industry has essentially predicated designs on a compromise of the these various competing considerations. However, there has remained an extent of the backbreak problem, top panel sealing problems, and/or overhead clearance limitations with existing doors of conventional track and roller design.